1. Field of the Invention
This invention relates to emission control of internal combustion engines. In particular, the invention relates to on-board monitoring of the efficiency of a catalytic converter installed on a vehicle to minimize emission from an engine.
2. Prior Art
It is known in the field that catalyst conversion relates to the oxygen storage capability of a catalytic converter. A properly operating catalyst dampens oxygen concentration fluctuations in the exhaust stream. One proposed system (see SAE paper 900062, Detection of catalytic converter efficiency loss using on-board diagnostic) employs two exhaust gas oxygen sensors (EGO), one before and one after the catalytic converter, to detect those changes in oxygen content before and after the catalytic converter. The system further employs test signals in a form of an air/fuel ratio swing on both sides of stoichiometry at predetermined rates or frequencies caused by fuel control system perturbations. By comparing the change in response patterns between pre- and post-catalytic converter EGO sensors, a determination can be made about catalytic converter efficiency. A deficiency of that method is that a fuel control system during test operates in an open loop, and air/fuel ratio tends to shift from stoichiometry. Test results also depend on two EGO sensors which may have different characteristics due to manufacturing tolerances or aging which may lead to additional errors. In particular, aging-. of the upstream EGO sensor can be very pronounced. Further, selected air/fuel ratio swing and frequency greatly influence results of the test.